For electrochemical measurements, a

For electrochemical measurements, a three-electrode setup comprising an Au-coated quartz crystal as the working electrode, a Pt-wire counter electrode, and a Hg/HgO/1 M KOH reference electrode was used in a Teon cell. Clean Pt surfaces were obtained by dipping the wire in aqua regia for a few seconds, rinsing it with copious amounts of water and annealing it with gas-O2 ame. A polymethyl pentane (PMP) tube encased the Hg/HgO reference electrode and a ceramic junction (ALS Co., Ltd) separated it from the alkaline reaction medium. The potential of the reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE) by measuring the equilibrium potential for the HER using a Pt wire working electrode in a 1 M H2-saturated KOH solution. The experiments were performed in both 1 M unpuried KOH and 1 M Fe-free KOH. The purication of the KOH (1 N standard solution, Merck KGaA, diluted with 18.2 MU cm H2O) was performed following a procedure already reported in the literature.31,34 For Co-based materials, the electrolyte was puried using Co(NO3)2$6H2O, while for Nibased materials the electrolyte was puried using Ni(NO3)2$6H2O. An anion exchange membrane (AGFA's ZIRFON PERL; comprising an open mesh polypropylene sulphide fabric symmetrically coated with a mixture of polymer and zirconium oxide) was used to separate the working and counter electrodes. The activity of the electrocatalysts was measured by linear sweep voltammetry at a scan rate of 5 mV s
1 .